Semiconductor devices and methods of fabricating the same

ABSTRACT

A MOS transistor includes a pair of impurity regions formed in a substrate as spaced apart from each other, and a gate electrode formed on a region of the substrate located between the pair of impurity regions. Each of the impurity regions is formed of a first epitaxial layer, a second epitaxial layer on the first epitaxial layer, and a third epitaxial layer on the second epitaxial layer. The first epitaxial layer is formed of at least one first sub-epitaxial layer and a respective second sub-epitaxial layer stacked on each first sub-epitaxial layer. An impurity concentration of the first sub-epitaxial layer is less than that of the second sub-epitaxial layer.

PRIORITY STATEMENT

This U.S. non-provisional patent application claims priority under 35U.S.C. §119 to Korean Patent Application No. 10-2011-0110109, filed onOct. 26, 2011, the entirety of which is hereby incorporated byreference.

BACKGROUND

1. Technical Field

The inventive concept relates to semiconductor devices and methods offabricating the same. More particularly, the inventive concept relatesto metal-oxide-semiconductor (MOS) transistors and methods offabricating the same.

2. Description of Related Art

There is an ever increasing demand for more highly integrated and higherperformance semiconductor devices. In this respect,metal-oxide-semiconductor (MOS) transistors constituting semiconductordevices are continuously being scaled down. For example, the thicknessof gate insulation layers of MOS transistors and the channel length ofthe MOS transistors are being minimized in an attempt to maximize thedensity of integration and performance of semiconductor devices. Inparticular, reducing the thickness of the gate insulation layers of theMOS transistors and the channel length of the MOS transistors results inan increase in the mobility of carriers (e.g., electrons or holes) ofthe MOS transistors and thus, an increase in the operating speed of thesemiconductor devices. However, small channel lengths of the MOStransistors sometimes give rise to a so-called short channel effect, andthin gate insulation layers may allow for excessive gate leakagecurrent. To suppress such a short channel effect, the channel of a MOStransistor may be formed with a high concentration of impurities.However, in this case, the carrier mobility in the channel region iscompromised by the impurities in the channel region because the greaterthe concentration of the impurities the lower the carrier mobility.Therefore, the switching speed of the MOS transistors is impeded eventhough the channel length is kept to a minimum.

SUMMARY

According to one aspect of the inventive concept, there is provided asemiconductor device including a substrate, a pair of impurity regionsspaced apart from each other in the substrate, and a gate electrodedisposed on the substrate at a location between the pair of impurityregions in a first direction, and in which each of the impurity regionsincludes a first epitaxial layer, a second epitaxial layer on the firstepitaxial layer, and a third epitaxial layer on the second epitaxiallayer, the first epitaxial layer is a stack of alternately disposedfirst and second sub-epitaxial layers (at least one each), and aconcentration of an impurity in material constituting the firstsub-epitaxial layer(s) is less than a concentration of an impurity inmaterial constituting the second sub-epitaxial layer(s).

According another aspect of the inventive concept, there is provided amethod of fabricating a semiconductor device which includes etching asubstrate to form a recess therein, forming a first epitaxial layer onwall surfaces delimiting sides and the bottom of the recess, andsequentially forming a second epitaxial layer and a third epitaxiallayer on the first epitaxial layer, and in which the first epitaxiallayer is formed by sequentially forming (at least one time) a firstsub-epitaxial layer and a second sub-epitaxial layer in such a way thata concentration of an impurity of material constituting the firstsub-epitaxial layer is less than a concentration of an impurity ofmaterial constituting the second sub-epitaxial layer.

According to still another aspect of the inventive concept, there isprovided a method of fabricating a MOS transistor which includes forminga gate insulating layer and a gate electrode layer on an active regionof a substrate, etching the active region of the substrate to formrecesses therein on opposite sides of a channel region, filling therecesses epitaxially with material whose lattice constant is greaterthan that of the channel region to form source/drain regions that applystress to the channel region and in which the filling of the recesses isperformed by forming a first epitaxial layer that extends along surfacesdelimiting the sides and bottoms of the recesses, to such a thickness asto leave upper parts of the recesses unfilled, then forming a secondepitaxial layer on the first epitaxial layer to such a thickness as toat least fill what remains of the recesses and subsequently annealingthe substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the inventive conceptwill become more apparent in view of the attached drawings andaccompanying detailed description of the preferred embodiments.

FIG. 1 is a cross-sectional view of a metal-oxide-semiconductor (MOS)transistor of a semiconductor device according to the inventive concept.

FIG. 2A is a graph of a boron concentration in a region ‘A’ of thedevice shown in FIG. 1.

FIG. 2B is a graph of a boron concentration in a region ‘B’ of thedevice shown in FIG. 1.

FIGS. 3, 4, 5A, 6, 7, 8 and 9 illustrate a method of fabricating a MOStransistor according to the inventive concept, and are each across-sectional of an intermediate structure formed during the method.

FIG. 5B is a timing diagram of supplying gases during a process offorming a first epitaxial layer shown in FIG. 5A.

FIG. 10 is an equivalent circuit diagram of an inverter including aP-channel MOS transistor according to the inventive concept.

FIG. 11 is an equivalent circuit diagram illustrating a static randomaccess memory (SRAM) cell including a P-channel MOS transistor accordingto the inventive concept.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Various embodiments and examples of embodiments of the inventive conceptwill be described more fully hereinafter with reference to theaccompanying drawings. In the drawings, the sizes and relative sizes andshapes of elements, layers and regions, such as implanted regions, shownin section may be exaggerated for clarity. In particular, thecross-sectional illustrations of the semiconductor devices andintermediate structures fabricated during the course of theirmanufacture are schematic. Also, like numerals are used to designatelike elements throughout the drawings.

It will also be understood that when an element or layer is referred toas being “on” or “connected to” another element or layer, it can bedirectly on or directly connected to the other element or layer orintervening elements or layers may be present. In contrast, when anelement or layer is referred to as being “directly on” or “directlyconnected to” another element or layer, there are no interveningelements or layers present.

Furthermore, spatially relative terms, such as “upper,” and “lower” areused to describe an element's and/or feature's relationship to anotherelement(s) and/or feature(s) as illustrated in the figures. Thus, thespatially relative terms may apply to orientations in use which differfrom the orientation depicted in the figures. Obviously, though, allsuch spatially relative terms refer to the orientation shown in thedrawings for ease of description and are not necessarily limiting asembodiments according to the inventive concept can assume orientationsdifferent than those illustrated in the drawings when in use.

Other terminology used herein for the purpose of describing particularexamples or embodiments of the inventive concept is to be taken incontext. For example, the terms “comprises” or “comprising” when used inthis specification specifies the presence of stated features orprocesses but does not preclude the presence or additional features orprocesses. Furthermore, the word or term “wall” is used synonymously attimes with the word “surface”.

An embodiment of a metal-oxide-semiconductor (MOS) transistor of asemiconductor device according to the inventive concept will now bedescribed with reference to FIGS. 1, 2A and FIG. 2B.

Referring first to FIG. 1, in this example, an isolation layer Fox isdisposed in a substrate 1 to define an active region. The substrate 1may be a single crystalline silicon substrate. Also, the substrate 1 maybe doped with impurities, for example, N-type impurities. A gateelectrode GT is disposed on the substrate 1. The gate electrode GT mayinclude at least one of a doped polysilicon layer and a metal containinglayer. A gate insulation layer Gox is interposed between the gateelectrode GT and the substrate 1. A first silicide layer SC1 may bedisposed at the top of the gate electrode GT. Furthermore, a spacer 22may be disposed on sidewalls of the gate electrode GT.

The MOS transistor also has a pair of spaced apart impurity regions SDin the active region of the substrate 1, for example. More specifically,in this embodiment, the impurity regions SD are disposed in respectiveones of recessed regions R of the substrate 1. Each of the recessedregions R includes opposing first sidewalls S1, opposing secondsidewalls S2 and a bottom surface S3 together delimiting a recess in theactive region of the substrate 1. Each of the first and second sidewallsS1 and S2 extends vertically, and the first sidewalls S1 adjoin thesecond sidewalls S2, respectively. Thus, each pair of adjoining firstand second sidewalls S1 and S2 may be seen as together constituting arespective side of the recessed region R.

Furthermore, in the example illustrated in FIG. 1, each first sidewallS1 is disposed above a second sidewall wall S2 and the second sidewallS2 may be connected to an edge of the bottom surface S3. Also, thedistance between the opposing first sidewalls S1 gradually increases ina direction toward the bottom surface S3, whereas the distance betweenthe opposing second sidewalls S2 gradually decreases in a directiontoward the bottom surface S3. Accordingly, the first and secondsidewalls S1 and S2 may have a (111) planar orientation and the bottomsurface S3 may have a (100) planar orientation.

The gate electrode GT is disposed on that part of the active regionwhich extends between the impurity regions SD.

Referring again to FIG. 1, each of the impurity regions SD includes afirst epitaxial layer L1, a second epitaxial layer L2 and a thirdepitaxial layer L3. The first epitaxial layer L1 is made up of at leastone pair of sub-epitaxial layers Lint and LB. Specifically, the firstepitaxial layer L1 includes at least one first sub-epitaxial layer Lintand at least one second sub-epitaxial layer LB which are alternatelydisposed one atop the other in a stack. The first sub-epitaxial layerLint is an intrinsic semiconductor epitaxial layer that is not dopedwith impurities. For example, the first sub-epitaxial layer Lint may bean undoped silicon-germanium epitaxial layer. The second sub-epitaxiallayer LB is an in-situ doped semiconductor epitaxial layer. For example,the second sub-epitaxial layer LB may be a silicon-germanium epitaxiallayer doped in-situ with boron impurities. In the case in which thefirst epitaxial layer L1 includes a plurality of first sub-epitaxiallayers Lint and a plurality of second sub-epitaxial layers LB, arespective first sub-epitaxial layer Lint is interposed between thesecond sub-epitaxial layers LB of each adjacent pair thereof in thestack, and another first sub-epitaxial layer Lint is interposed betweenthe lowermost second sub-epitaxial layer LB and the substrate 1. Also,in an example of this embodiment, another respective first sub-epitaxiallayer Lint is interposed between the uppermost second sub-epitaxiallayer LB and the second epitaxial layer L2.

Also, in this example, the second epitaxial layer L2 is an in-situ dopedsemiconductor epitaxial layer. For example, the second epitaxial layerL2 may be a silicon-germanium epitaxial layer doped in-situ with boronimpurities. The boron concentration of the second epitaxial layer L2 ispreferably within the range of about 2.2×10¹⁹ boron atoms/cm³. Moreover,in this case, the boron concentration of the second epitaxial layer L2is greater than that of the second sub-epitaxial layer LB.

In addition, the content of germanium in the second epitaxial layer L2may be greater than the content of germanium in the first epitaxiallayer L1. For example, the second epitaxial layer L2 may have agermanium content of about 35% in terms of atomic percent, and the firstepitaxial layer L1 may have a germanium content of about 25% in terms ofatomic percent. Also, the top surface of the second epitaxial layer L2may be located at a level above that of the top surface of the substrate1.

The third epitaxial layer L3 may be an in-situ doped semiconductor layeror an undoped semiconductor layer. For example, the third epitaxiallayer L3 is a silicon epitaxial layer doped in-situ with boronimpurities. Furthermore, a second silicide layer SC2 may be disposed oneach of the third epitaxial layers L3.

The MOS transistor shown in FIG. 1 may be a P-channel MOS transistor.

The portion of the substrate 1 under the gate electrode GT is a channelregion CR of the transistor. Each of the impurity regions SD adjacent tothe channel regions CR includes a silicon-germanium epitaxial layer, andthe lattice constant of the silicon-germanium layer is greater than thatof the channel region CR (a region of the silicon substrate 1). Thus, acompressive stress is applied to the channel region CR, such that thechannel region CR has a strained silicon lattice structure. Accordingly,carrier mobility (e.g., hole mobility) in the channel region CR is, ineffect, increased to improve the operating speed of the P-channel MOStransistor.

Referring now to FIGS. 1, 2A and 2B, the thickness of the firstepitaxial layer L1 on the first and second sidewalls S1 and S2 of therecessed region R is less than that of the first epitaxial layer L1 onthe bottom surface S3 of the recessed region R. For example, thethickness of the first epitaxial layer L1 in a region ‘A’ of the deviceshown in FIG. 1 is at most about 50 angstroms (Å), and the thickness ofthe first epitaxial layer L1 in a region ‘B’ of the device shown in FIG.1 is at least about 200 angstroms (Å). Also, referring to FIGS. 2A and2B, the impurity concentration C0 of the first sub-epitaxial layer Lintis relatively low, e.g., is less than about 1×10¹⁸ boron atoms/cm³. Inthe present embodiment, for example, the first sub-epitaxial layer Lintis an intrinsic semiconductor layer having an impurity concentrationclose to zero. On the other hand, the impurity concentration C1 of thesecond sub-epitaxial layer LB is substantially greater than that C0 ofthe first sub-epitaxial layer Lint. For example, the impurityconcentration C1 of the second sub-epitaxial layer LB is about 1×10¹⁹boron atoms/cm³.

Also, in FIGS. 2A and 2B, Tk1 indicates the thickness (a firstthickness) of the first sub-epitaxial layer Lint in the region ‘A’ ofthe device shown in FIG. 1, Tk2 designates the thickness (a secondthickness) of the second sub-epitaxial layer LB in the region ‘A’, Tk3designates the thickness (a third thickness) of the first sub-epitaxiallayer Lint in the region ‘B’ of the device shown in FIG. 1, and Tk4designates the thickness (a fourth thickness) of the secondsub-epitaxial layer LB in the region ‘B’. The first thickness Tk1 isgreater than the second thickness Tk2, and the third thickness Tk3 isgreater than the fourth thickness Tk4 in this embodiment. In addition,the third thickness Tk3 may be greater than the first thickness Tk1 andthe fourth thickness Tk4 may be greater than the second thickness Tk2.

That is, in this embodiment, the first sub-epitaxial layer Lint disposedon the bottom surface S3 of the recessed region R has a relatively lowimpurity concentration and has a thickness greater than that of thefirst sub-epitaxial layer Lint disposed on the sidewalls S1 and S2.Thus, during the fabrication of the MOS transistor and in particular,during an annealing process, the impurities (e.g., boron atoms) in thesecond epitaxial layer L2 will diffuse into the first sub-epitaxiallayers Lint. Accordingly, the impurity regions SD will have a gradedjunction profile in terms of their impurity concentration. Consequently,a junction leakage current of the impurity regions SD is minimized.

Meanwhile, the first sub-epitaxial layer Lint disposed on the sidewallsS1 and S2 of the recessed region R is thinner than the firstsub-epitaxial layer Lint disposed on the bottom surface S3. Thus, thefirst sub-epitaxial layer Lint disposed on the sidewalls S1 and S2hardly has any effect on increasing series resistance of the impurityregions SD. Consequently, any degradation of the current drivability ofthe MOS transistor by the first sub-epitaxial layer Lint is negligible.

Accordingly, a semiconductor device according to the inventive conceptoffers an improved leakage current characteristic and a high speedoperation.

A method of manufacturing a semiconductor device according to theinventive concept will now be described with reference to FIGS. 3 to 9.

Referring first to FIG. 3, an isolation layer Fox is formed in asubstrate 1 to define an active region. The substrate 1 may be a singlecrystalline silicon substrate or a silicon-on-insulator (SOI) substrate.The isolation layer Fox may be formed using a shallow trench isolation(STI) technique. A gate insulation layer Gox, a gate electrode GT and acapping layer 20 are then formed on the active region on the substrate1. The capping layer 20 may be formed of at least one of a silicon oxidelayer, a silicon oxynitride layer and a silicon nitride layer. The gateelectrode GT may be formed of a doped polysilicon layer. Next, a spacer22 is formed on sidewalls of the gate electrode GT. The spacer 22 may beformed of at least one of a silicon oxide layer, a silicon oxynitridelayer and a silicon nitride layer. Subsequently, a mask pattern M1 isformed on the substrate 1 to cover the gate electrode GT and theisolation layer Fox. The mask pattern M1 may be a hard mask formed of asilicon nitride layer or a photoresist pattern. The mask pattern M1 hasopenings that define the impurity regions SD of FIG. 1.

Referring to FIG. 4, the substrate 1 is etched using the mask pattern M1as an etch mask, thereby forming recessed regions R in the substrate 1.In this embodiment, the etching of the substrate to form the recessedregions R comprises sequentially performing dry and wet etchingprocesses. The wet etching process can be performed using an ammoniumhydroxide or a tetra-methyl-ammonium-hydroxide (TMAH) solution as anetchant. As a result of etching the substrate 1 by performing the dryetching process followed by the wet etching process, recesses are formedin the substrate 1 each being delimited by first sidewalls S1, secondsidewalls S2 and a bottom surface S3 (constituting a recessed region R).As was described above, the first and second sidewalls S1 and S2 have a(111) planar orientation, and the bottom surface S3 has a (100) planarorientation.

Referring to FIGS. 5A and 5B, a first epitaxial layer L1 is formed onthe first and second sidewalls S1 and S2 and the bottom surfaces S3using an epitaxial growth process. If the mask pattern M1 is aphotoresist pattern, the mask pattern M1 may be removed before the firstepitaxial layer L1 is formed. In this embodiment, the first epitaxiallayer L1 is formed using a modulation in-situ doping method.Specifically, the substrate 1 is loaded into a processing chamber,hydrogen (H₂), a germanium-based gas (GeH₄), gaseous hydrochloric acid(HCl) and dichlorosilane (SiH₂Cl₂, DCS) are continuously supplied intothe process chamber, and diborane (B₂H₆) is periodically supplied intothe process chamber as illustrated in FIG. 5B. Consequently, a firstsub-epitaxial layer Lint and a second sub-epitaxial layer LB arealternately and repeatedly formed in the recessed region R. That is, thefirst sub-epitaxial layer Lint is formed during each phase in which onlythe hydrogen (H₂), the germanium-based (GeH₄) gas, the gaseoushydrochloric acid (HCl) and the DCS are supplied into the processchamber. On the other hand, the second sub-epitaxial layer LB is formedduring each phase in which the diborane (B₂H₆) is additionally suppliedinto the process chamber. In some cases, silane (SiH₄) may be usedinstead of or in addition to the DCS.

Thus, the total time (tmt) for forming the first epitaxial layer L1, thetime (tm1) of each phase of forming the first sub-epitaxial layer Lintof the first epitaxial layer L1, the time (tm2) of each phase of formingthe second sub-epitaxial layer LB of the first epitaxial layer L1, andthe number (Nr) of times that first and second sub-epitaxial layers Lintand LB are both formed may satisfy the following:tmt=(tm1+tm2)×Nr

The ratio of tm2 to tm1 (tm2/tm1) is preferably within a range of about0.01 to about 0.5. For example, the first time (tm1) may be 1 second to50 seconds, and the second time (tm2) may be 0.1 seconds to 5 seconds.The diborane (B₂H₆) may be supplied at a flow rate of about 1 standardcubic centimeters per minute (sccm) to about 300 sccm while the diborane(B₂H₆) is injected into the process chamber.

Also, the total time of the phase(s) during which the secondsub-epitaxial layer(s) LB is/are formed using the diborane (B₂H₆) mayequal Nr×tm2.

In one example of this embodiment, a first sub-epitaxial layer Lint isformed as the uppermost layer of the first epitaxial layer L1. That is,after at least one first sub-epitaxial layer Lint and at least onesecond sub-epitaxial layer LB are formed, another first sub-epitaxiallayer Lint may be formed just before the second epitaxial layer L2 isformed. In this case, tmt=((tm1+tm2)×Nr)+tm2.

Also, the first epitaxial layer L1 tends to grow on the sidewalls S1 andS2 at a lower rate than on the bottom surface S3 because the sidewallsS1 and S2 have a (111) planar orientation whereas the bottom surface S3has a (100) planar orientation. Thus, the first epitaxial layer L1 isformed more thinly on the sidewalls S1 and S2 than on the bottom surfaceS3.

Referring to FIG. 6, second epitaxial layer L2 is formed on the firstepitaxial layer L1. As was mentioned above, the second epitaxial layerL2 may be a silicon-germanium epitaxial layer doped with boronimpurities. In the present embodiment, such second epitaxial layer L2 isformed by continuously supplying hydrogen (H₂), a germanium-based (GeH₄)gas, gaseous hydrochloric acid (HCl), dichlorosilane (SiH₂Cl₂, DCS) anddiborane (B₂H₆) gas into the process chamber. Also, the second epitaxiallayer L2 may be formed until it fills what remains of the recesses ofthe recessed regions R. The second epitaxial layer L2 may even be formedto protrude from a region of the substrate 1 (namely, from the activeregion).

As was described above, the first and second epitaxial layers L1 and L2may each be formed of a silicon-germanium layer and the substrate 1 maybe a silicon substrate. In this case, the lattice constants of the firstand second epitaxial layers L1 and L2 are each greater than that of thesubstrate 1. Thus, a compressive stress is applied to a region that willform the channel region (region ‘CR’ in FIG. 1) of the device. As aresult, the channel region CR will have a strained silicon latticestructure. Accordingly, carrier mobility is enhanced which facilitates ahigh speed operation.

Referring to FIG. 7, a third epitaxial layer L3 is formed on the secondepitaxial layer L3. The third epitaxial layer L3 may be formed bycontinuously supplying hydrogen (H₂), gaseous hydrochloric acid (HCl)and dichlorosilane (SiH₂Cl₂, DCS) into the process chamber. Also, duringthis process, a dopant gas, for example, diborane (B₂H₆), may or may notbe supplied into the process chamber. That is, as was described above,the third epitaxial layer L3 may be a silicon epitaxial layer dopedin-situ with boron impurities or an undoped silicon epitaxial layer.

Referring to FIG. 8, the mask pattern M1 is removed. The capping layer20 can then be removed to expose the gate electrode GT.

Referring to FIGS. 1 and 9, a metal layer Me is then formed over theentire (upper) surface of the substrate, and then the resultingstructure is annealed. As a result of the annealing process, a firstsilicide layer SC1 is formed on the gate electrode GT and a secondsilicide layer SC2 is formed on the impurity regions SD. The unreactedpart of the metal layer Me is then removed. In addition, during theannealing process, the impurities (e.g., boron atoms) in the secondepitaxial layer L2 diffuse into the first sub-epitaxial layer(s) Lint.

The MOS transistor (e.g., a PMOS transistor) according to the inventiveconcept may be employed by a logic circuit. For example, the PMOStransistor described above may be used in a complementarymetal-oxide-semiconductor (CMOS) inverter or a CMOS static random accessmemory (SRAM) cell.

FIG. 10 is an equivalent circuit diagram of an example of a CMOSinverter including a P-channel MOS transistor according to the inventiveconcept.

The CMOS inverter includes an NMOS transistor N1 and a PMOS transistorP1 serially connected to each other. More specifically, gates of theNMOS transistor N1 and the PMOS transistor P1 are electrically connectedto each other to act as an input terminal of the CMOS inverter, anddrains of the NMOS transistor N1 and the PMOS transistor P1 areelectrically connected to each other to act as an output terminal of theCMOS inverter. A source of the PMOS transistor P1 is electricallyconnected to a power supply Vdd, and a source of the NMOS transistor N1is grounded. The CMOS inverter thus inverts an input signal IN appliedto the input terminal of the CMOS inverter, and the inverted signal isoutputted through the output terminal of the CMOS inverter. For example,when the input signal has a logic level “1”, the output signal may havea logic level “0”. On the contrary, when the input signal has a logiclevel “0”, the output signal may have a logic level “1”.

FIG. 11 is an equivalent circuit diagram of an example of a CMOS SRAMcell including a P-channel MOS transistor according to the inventiveconcept.

Referring to FIG. 11, the CMOS SRAM cell includes first and secondaccess transistors Q1 and Q2, first and second driver transistors Q3 andQ4, and first and second load transistors Q5 and Q6. The first andsecond driver transistors Q3 and Q4 and the first and second accesstransistors Q1 and Q2 are NMOS transistors, while the first and secondload transistors Q5 and Q6 are PMOS transistors. Also, in this example,each of the first and second load transistors Q5 and Q6 is a PMOStransistor according to the inventive concept.

The first driver transistor Q3 and the first access transistor Q1 areserially connected to each other. A source of the first drivertransistor Q3 is connected to a ground line Vss, and a drain of thefirst access transistor Q1 is connected to a first bit line BL.Similarly, the second driver transistor Q4 and the second accesstransistor Q2 are serially connected to each other. A source of thesecond driver transistor Q4 is connected to the ground line Vss, and adrain of the second access transistor Q2 is connected to a second bitline /BL.

A source and a drain of the first load transistor Q5 are connected to apower line Vdd and a drain of the first driver transistor Q3,respectively. Similarly, a source and a drain of the second loadtransistor Q6 are connected to the power line Vdd and a drain of thesecond driver transistor Q4, respectively. The drain of the first loadtransistor G5, the drain of the first driver transistor Q3, and thesource of the first access transistor Q1 may thus constitute a firstnode. Similarly, the drain of the second load transistor Q6, the drainof the second driver transistor Q4, and the source of the second accesstransistor Q2 may constitute a second node. A gate electrode of thefirst driver transistor Q3 and a gate electrode of the first loadtransistor Q5 are connected to the second node, and a gate electrode ofthe second driver transistor Q4 and a gate electrode of the second loadtransistor Q6 are connected to the first node. Gate electrodes of thefirst and second access transistors Q1 and Q2 are connected to a wordline WL.

Accordingly, the first load transistor Q5 and the first drivertransistor Q3 are serially connected to constitute a first inverter, andthe second load transistor Q6 and the second driver transistor Q4 areserially connected to constitute a second inverter. Furthermore, thefirst and second inverters are cross coupled to constitute a latchcircuit.

According to the inventive concept as described above, there is provideda MOS transistor including a pair of impurity regions disposed at thesides of a channel region, and a lattice constant of the impurityregions is greater than that of the channel region. Thus, a compressivestress is applied to the channel region and as a result, the channelregion has a strained lattice structure. Accordingly, carrier mobilityin the channel region is enhanced to facilitate a high speed operationof the MOS transistor.

Furthermore, each of the impurity regions includes a first epitaxiallayer, and the first epitaxial layer includes at least one firstsub-epitaxial layer formed on sidewalls and a bottom surface of arecessed region of a substrate. That part of the first sub-epitaxiallayer formed on the bottom of the recessed region has a relatively lowimpurity concentration and is thicker than that part of the firstsub-epitaxial layer formed on sidewalls of the recessed region. Thus,during an annealing process in the method of fabricating the MOStransistor, the impurities (e.g., boron atoms) in the second epitaxiallayer diffuse into the first sub-epitaxial layer(s). Accordingly, theimpurity regions may have a graded junction profile in terms of theirimpurity concentration. Consequently, a junction leakage current of theimpurity regions may be minimized.

Finally, embodiments of the inventive concept and examples thereof havebeen described above in detail. The inventive concept may, however, beembodied in many different forms and should not be construed as beinglimited to the embodiments described above. Rather, these embodimentswere described so that this disclosure is thorough and complete, andfully conveys the inventive concept to those skilled in the art. Thus,the true spirit and scope of the inventive concept is not limited by theembodiment and examples described above but by the following claims.

What is claimed is:
 1. A method of fabricating a semiconductor device,the method comprising: etching a substrate to form a recess therein;forming a first epitaxial layer on interior sidewall surfaces of thesubstrate delimiting sides of the recess, and a surface of the substratedelimiting the bottom of the recess; and sequentially forming a secondepitaxial layer and a third epitaxial layer both on the first epitaxiallayer, wherein the forming of the first epitaxial layer includessequentially forming, at least one time, a first sub-epitaxial layer anda second sub-epitaxial layer in such a way that a concentration of animpurity of material constituting the first sub-epitaxial layer is lessthan a concentration of an impurity of material constituting the secondsub-epitaxial layer.
 2. The method of claim 1, wherein a total time(tmt) it takes to form the first epitaxial layer, a first time (tm1) ittakes to form a said first sub-epitaxial layer, a second time (tm2) ittakes to form a said second sub-epitaxial layer, and a number (Nr) oftimes a pair of respective ones of the first and second sub-epitaxiallayers is formed, satisfy the following equation:tmt=(tm1+tm2)×Nr.
 3. The method of claim 2, wherein a ratio of thesecond time (tm2) to the first time (tm1) is about 0.01 to about 0.5. 4.The method of claim 1, further comprising forming an additional saidfirst sub-epitaxial layer on a topmost one of the second sub-epitaxiallayers in the stack before the second epitaxial layer is formed, andwherein a total time (tmt) it takes to form the first epitaxial layer, afirst time (tm1) it takes to form a said first sub-epitaxial layer, asecond time (tm2) it takes to form a said second sub-epitaxial layer,and a number (Nr) of times a pair of respective ones of the first andsecond sub-epitaxial layers is formed, satisfy the following equation:tmt=((tm1+tm2)×Nr)+tm1.
 5. The method of claim 1, wherein the firstepitaxial layer is formed using a modulated in-situ doping technique. 6.The method of claim 1, wherein the forming of the first sub-epitaxiallayer comprises supplying (H₂), (GeH₄), gaseous (HCl) and at least oneof (SiH₂Cl₂) and (SiH₄) into a process chamber in which the substrate issupported.
 7. The method of claim 6, wherein the second sub-epitaxiallayer is formed by supplying (H₂), (GeH₄), gaseous (HCl), (B₂H₆) and atleast one of (SiH₂Cl₂) and (SiH₄) into the process chamber.
 8. A methodof fabricating a MOS transistor, the method comprising: forming a gateinsulating layer and a gate electrode layer on an active region of asubstrate; etching the active region of the substrate to form recessestherein on opposite sides of a channel region; filling the recessesepitaxially with material whose lattice constant is greater than that ofthe channel region to form source/drain regions on the opposite sides ofthe channel region, whereby the source/drain regions apply stress to thechannel region, wherein the filling of the recesses comprises forming afirst epitaxial layer that extends along sidewall surfaces delimitingsides of the recesses, and bottom surfaces delimiting the bottoms of therecesses, to such a thickness as to leave upper parts of the recessesunfilled, and forming a second epitaxial layer on the first epitaxiallayer to such a thickness as to at least fill what remains of therecesses; and subsequently annealing the substrate.
 9. The method ofclaim 8, wherein the forming of the first epitaxial layer comprisesepitaxially forming a first layer of an intrinsic semiconductor materialalong the sidewall and bottom surfaces to form a first sub-epitaxiallayer, and subsequently epitaxially forming a second layer of anintrinsic semiconductor material on the first layer, and doping thesecond layer with an impurity, to form a second sub-epitaxial layer onthe first sub-epitaxial layer and which the second sub-epitaxial layerhas a concentration of an impurity greater than that of the firstsub-epitaxial layer.
 10. The method of claim 9, wherein the forming ofthe second epitaxial layer comprises epitaxially forming another layerof an intrinsic semiconductor material on the first epitaxial layer, anddoping said another layer with an impurity at a concentration greaterthan that used to form the second sub-epitaxial layer.
 11. The method ofclaim 10, wherein the annealing forms a silicide layer at an uppersurface of the gate electrode.
 12. The method of claim 10, furthercomprising: epitaxially forming a third epitaxial layer on the secondepitaxial layer; and forming a metal layer on the third epitaxial layer,wherein the annealing forms a silicide layer on the third epitaxiallayer.